The invention relates to a Littrow grating with a multiplicity of parallel diffraction structures succeeding one another periodically, which are arranged on a support defining a base area and each incorporate a blaze flank inclined towards the base area substantially at the Littrow angle and a counter-flank, wherein the blaze flank and the counter-flank form at the apex of a diffraction structure an apex angle which is less than 90xc2x0. The invention also relates to uses for such a Littrow grating.
Such Littrow gratings are known commercially. They are used if a high reflection efficiency, i.e. a high diffraction efficiency in a Littrow arrangement, is required. An apex angle  less than 90xc2x0 results in the counter-flank, which is not exposed to light during the operation of the Littrow grating, being distanced from the path of the light beams impinging on the blaze flank. Calculations show that as a result an interaction in particular between the components of the incident light beams having TM polarisation and the counter-flank does not take place. Said interaction leads in the case of Littrow gratings with greater apex angles, at which the incident light beams run directly adjacent to the surface of the counter-flank, to an undesirable reduction in the reflection efficiency.
During the production of such known Littrow gratings a great deal of the material of the grating blank has to be removed on account of the apex angle  less than 90xc2x0, because the counter-flank xe2x80x9cdipsxe2x80x9d very far into the grating blank. Said need to remove a great deal of material during the production process makes the production of such known gratings complex and expensive.
It is therefore the object of the present invention to develop a Littrow grating of the kind mentioned in the introduction in such a way that it may be produced more simply and cheaply, without at the same time having to accept a reduction in the reflection efficiency.
Said object is achieved according to the invention by the fact that the counter-flank comprises at least two substantially plane area sections which, bordering one another and inclined relative to one another by an angle of inclination, extend parallel with the extension direction of the diffraction structure, wherein due to the inclination of the at least two area sections relative to one another the counter-flank all in all comprises a concave surface viewed from the light incidence side.
It is admittedly brought about by such a facetting of the counter-flank into at least two area sections that in the area of the apex of the diffraction structure the counter-flank rapidly removes itself from the path of the light beams (apex angle  less than 90xc2x0); however, because of the concave facetting and the resulting not so strongly inclined second area section the counter-flank does not dip as far into the grating blank as would be the case with a plane counter-flank. The amount of material removed during the production of the Littrow grating is therefore reduced, which simplifies the production and reduces its price.
Preferably the area sections exhibit, measured normal to the extension direction of the diffraction structures, a width ratio of 0.5 to 2. With such a width ratio, with which one area section is at most twice as wide as the other, the area of the counter-flank in which the two area sections border one another, with given angle of inclination and given apex angle, is relatively far removed from the light path of the incident light beams. The interaction of the incident light beams with the counter-flank is thereby optimally small.
The angle of inclination may lie in the range from 90xc2x0 to 150xc2x0. With such a choice of the angle of inclination a good reduction in the material which has to be removed to produce the Littrow grating is obtained.
Preferably the Littrow grating consists of quartz glass. Such materials may be worked by reactive ion beam etching (RIBE) or by reactive ion etching (RIE) and are therefore considered for the holographic production of the diffraction structures according to the invention. Alternatively a crystalline material may be used as the material for the Littrow grating, such as e.g. silicon, wherein the crystal surface may then additionally be aligned for the crystallographic orientation of the crystal in such a way that a privileged direction is obtained for the working, e.g. by anisotropic chemical etching with KOH. Said privileged direction may be exploited e.g. for the setting of the angle of inclination or of the apex angle. If the diffraction grating consists of doped quartz, the holographic production process is further simplified, since said material is because of its advantageous expansion characteristics well suited both for such a production process and for use.
The Littrow grating may comprise a coating increasing the reflectivity. The reflection efficiency of the Littrow grating is thereby increased.
In the case of the use of a coating increasing the reflectivity the latter is preferably an aluminium coating. Such a coating is relatively cheap and exhibits a high achievable reflecting power.
Alternative possibilities of increasing the reflection efficiency by the choice of material are obtained if instead of a coating increasing the reflectivity the Littrow grating comprises a dielectric layer system, e.g. a plurality of mutually succeeding layers, e.g. of Al2O3 (high refractive index) and MgF2 (low refractive index) or of LaF3 (high refractive index) and MgF2 (low refractive index), wherein the layer sequence is so chosen that a reflective layer of high efficiency results
The blaze flank may exhibit a minimum width of g cos (theta), measured normal to the extension direction of the diffraction structures, wherein g designates the grating period of the Littrow grating and theta the Littrow angle. Such a width guarantees that the whole bundle of incident light beams may be reflected by the blaze flanks. If the blaze flank has exactly the width according to the specified calculation formula, a Littrow grating may be obtained with minimal removal of material.
It is a further object of the present invention to specify uses for the Littrow grating according to the invention in which the described advantages of the Littrow grating are put to good use. Said object is achieved by the preferred uses listed below:
A preferred use of the Littrow grating is one in a diffraction order of the incident light wavelength above or equal to the 15th diffraction order. With such a high diffraction order the dispersion of the Littrow grating for the selection of the reflected wavelength is advantageously high. At the same time the demands made of the production of the Littrow grating are not so high, since the grating period is proportional to the diffraction order.
An advantageous area of use of the Littrow grating is the diffraction of UV light with a wavelength which is less is than 250 nm. Such UV light sources are used in particular in projection lithography for the production of semiconductor structures in chip manufacture. With a Littrow grating according to the invention a wavelength which is narrow-band and efficient may be selected for such a projection light source.